Huanglongbing (HLB), commonly known as citrus “greening” disease, is one of the top three most damaging diseases of citrus in Africa, America and Asia. HLB is naturally transmitted by psyllids, and experimentally by grafting or dodder (Cuscuta spp.). The disease was shown to be graft-transmissible in 1956 (Lin, 1956) and therefore it was thought to be caused by a putative virus. However, in 1970, sieve tube restricted bacteria were discovered in affected trees. First thought to be mycoplasma-like (Laflèche and Bové, 1970), they were soon recognized as walled bacteria (Saglio et al., 1971; Bovd and Saglio, 1974) of the Gram negative type (Gamier et al., 1984) and finally shown to be species of alpha proteobacteria (Jagoueix, et al. 1994). Two species were recognized: Candidatus Liberibacter asiaticus (Las) for the disease in Asia and Ca. L. africanus (Laf) for the disease in Africa.
In 2004, when HLB was seen for the first time in the Americas and more precisely in Sbo Paulo State, Brazil, two liberibacter species were identified: (i) a new species, Ca. L. americanus (Lam), infecting most of the affected trees, and (ii) the known Asian liberibacter, Las, present in a minority of trees (Teixeira et al., 2005). All three citrus liberibacters are uncultured and phloem-limited. That is, these bacteria live in plants exclusively within living plant phloem cells. Las is the most widely distributed by far. Today, HLB caused by Las has been identified in states ranging from Florida, Louisiana, and California.
With no effective treatment options available in the market, there is a growing demand for new technologies to combat its spread.